Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Startup Stash - A curated directory of 400 resources & tools for startups
WorkPatterns - The modern manager's toolkit: 1:1s, feedback, & recognition
Content Marketing Stack - A curated directory of content marketing resources
SlimPlanner - Team scheduling, resource planning, task management
StartupResources.io - Tightly curated lists of the best startup tools